Dave I dont know that I am afraid. I would imagine it would depend on which of the three versions you go for, the lite version would definitely work out dearer as it will cost less than Mach costs now, that is unless there is a free upgrade to it. Its unlikely that anyone with Mach3 would want to downgrade to lite version, then again I suppose they might as maybe some dont use many features of Mach and there may be some benefits to them over Mach3
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Mach 4 Pro is to be priced just a bit more than Mach3 is at the moment so not sure whether the upgrade cost would just be the extra over what Mach3 is, if that was the case then there would be no more expense buying Mach3 now and upgrading later.
The there is the expert version, its mainly aimed at the commercial user and will have support contracts so unlikely many here will go for that but I would imagine if one went that way then it likely the upgrade would be the cost of that version minus the cost of Mach3.
All this is just my thinking and none of it is fact so it probably wasnt worth reading

Hood